Three-phase motors including a stator with stator coils and a rotor with a permanent magnet have been used as a driving source of electric devices. Rotation of the rotor is controlled by controlling attraction force and repulsion force acting between magnetic flux generated when applying current to the stator coils and magnetic flux generated by the permanent magnet. Such control method needs determination of position of the rotor and appropriate current supply to the stator coils depending on a position of the rotor. Although it is possible to use a rotation sensor for detecting accurate position of the rotor, this method leads to a cost increase. Therefore, for detecting the position of the rotor without using the rotation sensor, several technical approaches have been reviewed for example, as disclosed in JP2005-110345A (which will be hereinafter referred to as Patent reference 1).
The Patent reference 1 discloses a control device for activating a sensorless-type motor for driving a hydraulic pump. When hydraulic oil temperature measured by a temperature sensor is higher than a first predetermined temperature, the sensorless-type motor is activated with open-loop control with a first starting commutation frequency, and then, after the speed of rotation reaches a predetermined speed, the sensorless-type motor is controlled by closed-loop control. When hydraulic oil temperature is lower than the first predetermined temperature, the sensorless-type motor is activated and operated with open-loop control with the second starting commutation frequency, which is equal to or less than the first starting commutation frequency.
As described above, the control device disclosed in the Patent reference 1 is intended to activate the sensorless-type motor based on measured temperature of the hydraulic oil. When the hydraulic oil temperature is lower than the predetermined temperature, the commutation frequency is decreased. Since electric power is supplied from an electric generator or a battery to the sensorless-type motor, output voltage may fluctuate. For example, when the output voltage increases, the sensorless-type motor rotates rapidly and may exceed the desired angle of rotation. On the other hand, when the output voltage drops, the speed of rotation of the sensorless-type motor becomes slower and a time period for detecting the position of the sensorless-type motor becomes longer, therefore prolonging a start time.
A need thus exists for a motor control device, which is not susceptible to the drawback mentioned above.